1. Field of the Invention
This invention relates to the field of sound generation and modification. More specifically, the invention comprises a rotary transducer where the pitch of rotating vanes is used to create or modify pressure waves. The invention includes additional features to increase the output of the transducer in the upper portion of its frequency response range.
2. Description of the Related Art
Rotary sound transducers convert non-acoustic input energy into acoustic output energy by varying the pitch of rotating vanes. The vanes typically rotate in a fixed arc around a hub. The pitch of the vanes is varied as they rotate in order to create the acoustic output energy. One example of such a device is disclosed in U.S. Pat. No. 2,304,022 to Sanders (1942) (hereinafter “Sanders”). Sanders discloses a sound producing apparatus that resembles an electric fan. Cyclical electrical energy is fed into an electromagnet in the invention's hub. The input energy cyclically varies the pitch of the vanes—thereby producing sound waves at a desired frequency.
Another type of rotary transducer is disclosed in my own prior patent application (U.S. patent application Ser. No. 10/442,852). My prior application uses a swash plate to vary the pitch of the rotating vanes in a manner reminiscent of the mechanism used to vary the pitch of a helicopter's main rotor. FIGS. 1-3 refer to my prior design.
FIG. 1 shows the main components of the prior art rotary transducer. A pair of vanes is driven in the arc shown by shaft 30. A motor within housing 26 spins shaft 30. Swash plate 36 actuates two linkages 40 connected to a pitch-actuating mechanism on each vane. The reader will observe that moving the linkages will cause the deflection of the two vanes 34 so that the angle of attack of each vane (relative to the air flowing over its leading edge) is increased or decreased. The angle of attack of the two vanes is changed in unison.
Swash plate 36 translates in a direction that is parallel to the central axis of shaft 30. The swash plate is urged toward the vanes or away from the vanes by the motion of voice coil 20. Voice coil 20 is an electromagnetic device such as used in a common audio speaker. The voice coil is suspended in a neutral position by suspension spider 22 (which is also commonly used in audio speakers) or held in the neutral place by the influence of the air load on the leading and trailing edges of the vanes. Wire bundle 42 includes the wires used to provide electrical power to the motor that rotates the vanes and other wires used to provide the input for the motion of the voice coil.
FIG. 2 shows a sectional view through the prior art device with the vanes in the “neutral” position. In this position the vanes have a zero angle of attack and no pressure waves are produced (other than a small cyclical output caused by the flat vanes “cutting” through the air). Motor 28 provides the driving torque for shaft 30. Voice coil assembly 12 includes the components that move the swash plate. Magnet 18 is held between back plate 14 and front plate 16. Voice coil 20 moves linearly (left and right in the orientation of the view) as a magnetic field is applied by center pole assembly 24 under the influence of magnet 18.
Conventional rotary bearings 32 support the rotating shaft. Bearing assembly 38 is a thrust-type bearing. It allows swash plate 36 to rotate with respect to voice coil 20 while also transmitting a linear force. Although the mechanism shown is reminiscent of that used in a helicopter's main rotor, the reader will note that the pitch of the two vanes is not varied independently but always in unison. Thus, using helicopter terminology, the simple swash plate is able to vary the “collective” pitch but is unable to create cyclical variations customarily produced by tilting the swash plate in a helicopter.
FIG. 3 shows the same mechanism with voice coil 20 pushed away from the neutral position by the application of electromagnetic force. The voice coil and swash plate have been urged to the left in the orientation of FIG. 3. Thus, the two linkages 40 have also been pushed away from the neutral position and the two vanes 34 have been pitched as shown (in opposite directions). The result is that the vanes are given a substantial angle of attack.
A transducer such as shown in FIGS. 1-3 is able to produce low frequency sound waves without requiring a large and heavy conventional transducer. The shaft rotates the assembly at a speed which is often much higher than the frequency of the desired sound waves. As an example, the shaft might be rotated between about 600 and 1000 RPM, while the transducer might be used to generate sound waves in the range of 20 Hz to 100 Hz. Sound in this range may be effectively produced using a rotary transducer having an overall diameter of about 8 inches. In contrast, a 15 inch to 18 inch cone speaker will often be needed to produce a 20 Hz output. An enclosure for such a speaker will add considerable bulk and mass as well.
By virtue of the rotational speed of the blades and the swept air of the blades for each cycle at very low frequencies the rotary transducer offers a significant impedance match advantage with air or fluids in comparison to a moving cone or piston.
The rotary design can be used in an enclosure or box where the back wave pressure is captured and the transducer becomes a monopole. Because the rotary design has a significantly improved impedance match with the air, it can also be used as a dipole for low frequency sound reproduction.
Of course, when operated as a dipole, air within the positive pressure generated on one side of the plane of rotation has an easy path of travel to the negative pressure on the opposite side of the plane of rotation. This forms a sort of “short circuit” for dipole operations. The effect of the “short circuit” in dipole operation varies with frequency. The transducer is generally rotated at a relatively constant speed. Thus, the “swept area” of the vanes is constant. For low frequency inputs, the output amplitude is good. A significant amplitude “roll off” is experienced for higher frequencies, however.
At extremely low frequencies one can achieve one or more full revolutions of the drive shaft per pitch cycle of the vanes. As the input frequency is reduced, the impedance match with the air improves due to the increase in swept area. Conversely, at higher frequencies the swept area is reduced in comparison to the rotational velocity and each pitch cycle or oscillation may only consume a small portion of a full revolution of the drive shaft. This reduction in effective area and shorter wavelengths result in a 12 dB per octave decrease in output amplitude for increasing input frequency with the prior art construction.
The “roll off” with increasing frequency is exactly the opposite of what occurs with a conventional cone-type loudspeaker. Such speakers are driven by a linear actuator (a voice coil) connected to a cone or “piston”. As the input frequency to the voice coil increases, the wavelengths decrease relative to the physical dimensions of the piston and the impedance match with the air becomes more favorable. Since the wavelength of sound decreases with increasing frequency and the net radiating area of the piston is constant, the impedance match of the cone with the air is improved.
Two factors dictate the “roll off” a rotary vane transducer experiences with increasing input frequency. The first factor is loss of the impedance match with the air as the frequency is increased. The second factor is the inertia of the actuating mechanism and the vanes themselves which requires more force from the actuator to maintain the same acoustic output. It is therefore desirable to produce a rotary transducer that retains the ability to produce low frequency sound while reducing the “roll off” phenomenon inherent in the prior art devices.